1. Field of the Invention
Embodiments of the present invention generally relate to methods for forming semiconductor devices. More particularly, embodiments of the present invention generally relate to methods for preparing and forming metal contacts on a semiconductor substrate.
2. Description of the Related Art
Metal gates or contacts typically include a doped silicon surface, one or more barrier layers, one or more liner layers and bulk metal to complete the gate structure. The cleanliness of the substrate surface between layers is critical for reducing contact resistance and hence, optimal device performance. For logic devices, the contact is usually a silicide, such as nickel silicide (NiSi), cobalt silicide (CoSi2), or titanium silicide (TiSi2). Nickel silicide is getting more popular for smaller geometries, e.g. geometries having aspect ratios of about 10:1 or smaller, because NiSi is widely available and has a lower resistivity and lower contact resistance compared to other metal silicides.
In a typical fabrication process, the metal silicide is formed on a substrate in one vacuum environment and the substrate is transferred to another vacuum environment for further processing. As a result, the substrate can be subjected to oxidative conditions during the transfer. A clean process is typically conducted prior to the liner/barrier deposition to remove any oxides on the silicide surface which formed during transfer and exposure to the oxidative environment.
Conventional clean processes utilize physical etch techniques, i.e. sputtering, or chemical etch techniques. Sputtering techniques can damage the underlying surface due to resputtering of oxide onto the silicide surface. Sputtering techniques can also change the contact hole geometry due to the physical bombardment of ions on the substrate surface. For example, the contact opening can become widened or tapered which is sometimes referred to as “faceting.” Conversely, chemical etch processes tend to cause less physical damage to the substrate surface, but can alter the surface composition. Such changes to the surface composition can lead to higher contact resistance. For example, the stoichiometry of the metal silicide can change when silicon atoms are removed during the clean process, thereby providing a metal rich silicide layer that can have a higher contact resistance.
The silicide integrity can recover by allowing either the metal atoms to migrate to the underlying regions or the silicon atoms to migrate from the underlying regions. Migration can be dependent on the composition of the silicide, the thickness of the silicide and temperature. Depending on such factors, migration can be very time consuming, on the order of 20 minutes or more, which is unacceptable to industry throughput requirements. Moreover, such migration can change the depth of the underlying regions (e.g. heavily doped isolation regions underneath the silicide regions) making those regions shallow, which can result in high leakage of current.
Therefore, there is a need for a new process for forming metal gates that can quickly recover the silicide integrity without altering device performance.